Steam generator with forced passage of the operating medium



April 4, 1939. H, RABE 2,153,001

STEAM GENERATOR WITH FORCED PASSAGE OF THE OPERATING MEDIUM Filed Nov. 6, 1936 Patented Apr. 4, 1939 PATENT OFFICE STEAM GENERATOR WITH FORCED PAS- SAGE OF THE OPERATING MEDIUM Heinz Babe, Berlin-Spandau, Germany, aslignor to Siemens-Schnckertwerie Aktiengeaelllchaft,

Berlin-Siemensstadt, of Germany I Germany, a corporation Application November 6, use. sci-a1 No. 109,435

In Germany November '1, 1935 9Claims.

furnace by means of a temperature impulse de rived from the steam issuing from the boiler. Such a control has the disadvantage'in that it more or less lags behind the momentary operating conditions of the boiler thus producing hunthand, the changes in temperature occurring in ing" in the control operations of the boiler. The lagging of the control is due to the. fact that a change in heating can always manifest itself at the end of the boiler as a change in the steam temperature only when the portionof the operating medium entering the boiler at the moment of the change in heating has reached the end of the boiler. Since the passage of the operating medium through the boiler takes, as a rule, some minutes, the control impulse arrlves, cons equently,' always too. late. 4

So far as I am aware, no expedients have been suggested which would remove this drawback in a satisfactory manner. By displacing the point at which the impulse is derived towards the feed point opposite to the direction of flow of the operating medium it is hardly possible to improve the operating "conditions of the boiler, since the point from which the impulse is derived can only be displaced back to the point, where the superheating begins. The heating surface for evaporation adjacent to this point cannot, as is well known, be utilized for the transmission of' temperature impulses, since the operating medium enclosed in this heating surface has always the same temperature independently of the load of the boiler, i. e., the temperature of the saturated steam, corresponding to the operating pressure. There remains the possibility of deriving the impulse from the preheater. But this possibility cannot be taken advantage of, since on the one the preheater are comparatively slight and, on the other hand, because the impulse arrives too soon and, consequently, too great a portion of the boiler lying behind the point from which'the impulse is derived could not be controlled. It. is self -evident that also a combined impulse composed of an impulse derived from the preheater and one derived from the superheater cannot be taken advantage of, since also in an impulse thus composed the one or the other impulse predominates, and, as above described, is not transmitted at the proper time. v

The invention provides a fundamentally novel method of obtaining within a shorter time interval a temperature impulse'in response to a change in the heat supplyto the boiler. According tothe invention, a-,secondary heating surface is allotted to the boiler heating surface proper,

(|. ass-' in which secondary surface the time required for the passage of the medium through it is shorter than that required for the operating medium to traverse the main surface and that the changes in temperature of the medium flowing in the secondary heating surface are utilized for controlling the boiler. By this arrangement the important advantage is obtained that it is possible to reduce at will the factor which had hitherto delayed the transmission of the temperature impulse; i. e., the time required for the passage of the operating medium.

. The secondary heating surface which may be formed of a single tube of comparatively small,

diameter may be exposed at any point to the hot gases in the boiler and be given any suitable form. In this case the changes in the heat supply to the boiler willnot, however, always influence the boiler heating surface proper and the secondary heating surface to the same extent; 1. e., the temperatures of the steam prevailing at the end of the secondary heating surface will not always represent a true picture of the temperatures at the end of the boiler. However, that does not exclude the possibility of the furnace control by the temperature impulses derived from the secondary "qheating surface. To obtain a proper control, it is only. necessary tocorrespondingly correct the secondary impulses and to cause these corrected impulses toact upon the furnace control. To this end, any of the various means known in the art may be employed such as cams, stepped resistances. and the like.

As a rule, the results obtained thereby are, however, not satisfactory. A solution of the problem of attaining a more satisfactory and proper control consists, according to the present invention, in designing and arranging the secondary heating surface in such a manner with respect to the to the boiler are proportional to the temperature changes-at the end of the main heating surface. To accomplish this, I propose to design and arrange the two heating surfaces so that both extend at least approximately through the same heating zones in the boiler. Under certain circumstances, it is, however, not possible to even then attain a suflicient proportionality of the changes in temperature at the end of the main heating surface and the secondary heating surface, and it is therefore advisable also to make in all heating zones the ratio between portions of the secondary heating surface and the main heating surface, which are ex'; used to like temperature, at least approximately equal.

In order to attain at the end of the secondary surface as wide a change in temperature as possi- 'ble, it is further proposed according to the inventionto feed the secondary heating surfacewith a liquid, which may, if desired, also be a portion of'the feed water, in such a manner that the liquid traverses said heating surface without developing steam. If a development of steam were permitted the secondary heating surface would have along a certain section within which evaporation occurs a constant temperature of the fluid to be measured, 1. e., the secondary heating surface would contain a section which is necessary for the present purpose. At this point it might be stated that it is advisable, under certain circumstances, to feed the secondary sur-' face with oil in order to obtain at the end of the section to be measured temperatures as high as possible and, therefore, great changes in temperature. Instead fof a liquid also a gas may, however, be caused to pass through the secondary heating surface, which may be advantageous, since the resistance to flow of a gas is smaller than that of a liquid so that the time required for the operating medium to flow through the secondary heating surface may be considerably shortened.

The feeding of the secondary heating surface may be either in such a manner that it is always proportional to the feeding of the main heating surface varying upon fluctuations of the boiler load or that it is maintained always unaltered and independent of the load fluctuations of the boiler.- In' the second case it must be observed that updn "load fluctuations of the boiler changes in temperatures are brought about at the end of the secondary heating surface, whereas the temperature at the end of the main heating surface remains constant in case with a change in the boiler load the supply of heat and operating medium has been at the same time varied accordingly. Fromthe temperature measuring device located at the end of thesecondary heating surface a control impulseacting on the furnace would be, consequently, transmitted which would erroneously vary the supply of heat.

This may be, however, avoided in a simple manner by causing a compensation impulse to act on the. governor which is automatically re-' leased upon fluctuations of load and whose ma nitude of. action corresponds to the magnitudeof the change in load. ,If a resistance thermometer is employed as temperature measuring device at the end of the secondary heating surface and if the governing device is constructed in a manner well known so that by the use of a Wheatstone bridge the resistance in one branch of the bridge and, therefore, the equilibrium ofthe bridge is disturbed upon changes in temperature, the above-given directions may be realized in a simple manner by preventing a disturbance of the balance of the bridge, which is brought about by inserting in another branch of the, bridge a resistance which upon changes in load is correspondently varied. This arrangement willv be hereinafter more fully described in connection with the embodiment of the invention.

Instead of a Wheatstone bridge any other'electrio or hydraulic control device may also be employed. If, for instance, a control device having a contact pointer be employed, whose deflectionsv correspond to the changes in temperature at the end of the secondary heating surfaces, 9. false control with changes in load of the boiler may be prevented in a simple manner by shifting with stationary counter-contacts cooperating with the pointer contact in the one or the other direction.

A false control may under certain circumstances also occur if the temperature of the medium fed into the main heating surface or into the secondary heating surface varies. If necessary, also both of these influences must he, therefore, considered. When using resistance thermometers at the feeding point of both heating surfaces as well'as a Wheatstone bridge this may be accomplished in a' simple manner in the branches of the bridge by inserting the corresponding resistance thermometers whose resistances vary in accordance with the temperatures prevailing at the feeding points.

When using a hydraulic control device the changes in temperature ofthe medium fed may be controlled by correspondingly designed intermediate gears of the, control device. In the case of a control devicewith a contact pointer this may be also accomplished in a simple manner by displacing accordingly the pair of normally stationary counter-contacts cooperating with the pointer contact in one or the other direction. If in this case, as above described, a false control is to be prevented at the same time with changes in load of the boiler, the driving mechanism which serves to displace the pair of counter-contacts must be designed as multiple differential gear which permitsa displacement of the contact pair without mutual influence in such a manner as is required by a changein load or a change in one or the other temperature of the media fed.

The control devices may in all cases be designed with or without a return control mechanism. If control devices without return control mechanism are employed the temperatures prevailing at the end of the secondary heating surface are brought back to their initial value from the furnace side of the boiler. If the return control occurs so sluggish as to cause hunting during the control it is advisable to locate the return control mechanism in the control device itself. In this case the return control mechanism may be rigid or flexible. How the return control of a hydraulic control device or a control device provided with a pointer contact maybe attained in connection with a Wheatstone bridge is well known-in the art and is of o importance to the invention. It will, therefore, not be necessary to deal with this point.

However, the objection might be raised that the new control is not economical owing to the considerable amount of heat supplied to the secondary heating surfaceand, therefore, withdrawn from the main heating surface. If really appreciable losses should arise from the novel measuring method-for the just-mentioned reasons, it is only necessary to supply the medium issuing from the secondary heating surface to a heat exchanger inwhichthe heat supplied to the sec-- ondaryheating surface is regained so that it may be utilized for any suitable purpose.

The accompanying drawing shows an embodiment of my invention in diagrammatic form.

Referring to the drawing the boiler heating surface or the main heating surface which serves to produce the useful steam is denoted by the numeral I and the secondary heating surface by the numeral 2. Both heating surfaces are heated by a common boiler furnace, schematically shown .at 3. As is indicated by the arrows 4 to 6, the

two heating surfaces I and 2 aresimilarly heated changes inload of the boiler, the two normally in each ofthe furnace zones indicated by these arrows. The heating surface is fed by a pump 1 and the secondary heating surface 2 by a pump 8. The steam produced by the heating surface I issues from the conduit 9. The fluid issuing from the secondary heating surface 2 flows into the conduit III. In this conduit is arranged a tem-' perature measuring device II, also in the supply conduit l2 leading to the heating surface 2 and in the supply conduit I3 leading to the heating surface 1 are inserted temperature measuring devices I and I5 respectively. The secondary heating surface is inserted in a circuit formed by the conduits l6, l1, l8 and IS in which a heat exchanger is arranged.

A Wheatstone bridge is employed for nieasuring the temperatures at H and M. The four branches of the bridge are provided with four resistors 2|, 22, 23 and 24 of which 2|, 22 and 23 are part of the measuring devices H, l5 and M respectively. The apparatus is so designed that the resistances of the resistors 2 I, 22, and 23 vary in accordance with the changes in temperature at the points I I, I4 and I5 at which the temperature is measured. The resistance 24 may be varied .by a sliding contact 25 which is automatically displaced as soon as the load of the boiler increases or decreases. The diagonal 26 contains an instrument 21 which, in reaction to a compensating current flowing in this diagonal, controls, for instance bya relay, the speed of a motor 28 for driving the fuel supply apparatus .29 and, if desired, the speed of a blower (not shown) for the air of combustion. Besides, the motor 28 may be controlled as is indicated by the dash line 30 by a so-called master governor not shown here, i. ct, a governor which permits to vary the load of the boiler by controlling at the same time the devices for the supply of the fuel,

air of combustion and the feed water.

The operation of the system is as follows As long as the system is in a state of equilibrium also the bridge isin a state of equilibrium, that is to say, no current flows in the diagonal 26. The

fuel supply apparatus 29, the feed pump 1 and the apparatus for the supply of the air of combustion, not shown, rotate at a given normal speedand the boiler supplies a corresponding.

amount of steam of a certain temperature.

If now, for instance, the output of the boiler increases in'response to the reaction of the master governor to increased load requirements the following occurs when feeding the secondary heating surface 2 at a constant rate: Assuming that the master governor operates in such a manner that the supply of fuel, operating medium and air of combustion varies in such a proportion that the steam issuing from the conduit 9 has always the same temperatura then an increase in temperature would nevertheless result at the point ll atwhich the temperature is measured, owing to the changed heat supply to the sec-1 ondary heating surface. This increase in temperature does not, however, influence the fuel supply apparatus 29-, since with the increase in load the resistance 24 is varied by the assumed displacement-of the sliding contact 25 to such an extent that the equilibrium of the bridge is not disturbed by the change in the resistance 2|. Consequently, the pressure of the boiler may be regulated to a higher or lower value without there occurring a false control of the furnace from the measuring point H.

' medium fed into the secondary heating surface 2 decreases or increases. If the temperature, for

instance, increases at the point ll it also increases at the point II. The increase in' temperature at these two points does not bring about a response of the fuel suppiyapparatus 29, since both resistances 2| and 22 which vary in accordance with the changes in temperature at thepoints II and H are inserted at such points in the bridge circuit that simultaneous increase'or decrease in temperature at the points II and it compensate one another, that is to say, no current flows in the bridge diagonal 26,

An actual response of the fuel supply apparatus is effected only in two cases. One case occurs when the heating of the boiler varies for any reason in the state of equilibrium of the boiler, and the other case occurs when the temperature of the medium fed into the main heatillg surface I' increases or decreases at the poin The operation of the governor in the first-mentioned case has already been described and need not, therefore, be further explained.

In the second case also a response iseffected in a similar manner as in the first caseif the resistor 23 whose resistance varies in accordance with the temperature prevailing at point l4 disturbs the equilibrium of the bridge. In this manner a compensating current flows in the bridge diagonal 26 which accordingly adjusts themotor .28 and the fuel supply apparatus 25 through the tially greater than the flow speed of said oper-" ating medium through thexmain heating surface, and means responsive to the fluid temperature ,variations in said secondary surface for controlling the heat supply to the generator.

2. In a steam generator with forced passage of the operating medium and having a heat supply producing several heating zones of different temperatures, a main heating surface for said medium extending through the several heating zones and a secondary heating surface traversed bya suitable fluid and extending at least approximately through the same heating zones as the main surface, the area ratios between the main and the secondary heating surfaces being at leastapproximately equal in the several heating .zones, the flow speed of the fluid through said secondary surface being substantially greater than the flow speed of said operating medium through the main heating surface, and means responsive to the fluid temperature variations in said secondary surface for controlling the heat supply to the generator.

3. In a steam generator with forced passage of the operating medium and having a heat supply producing several heating zones of different temperatures, a main heating surfacefor said medium extending through the several heating zones and a secondary heating surface traversed by a suitable fluid andlex'tending at least approximately through the same heating zones as the main surface, the fluid traversing said secondarysurface being in liquid state throughout the entire length of said surface and its flow speed being Substantially greater than the flow speed of said operating medium through the main heating surface, and means responsive to the fluid temperature variations in said secondary surface for controlling the heat supply to the generator.

4. In a steam generator with forced passage of the operating medium and having a heat supply producing several heating zones of different temperatures, a main heating surface for said medium extending through the several heating zones and a secondary heating surface traversed by a suitable liquid and extending at least approximately through the same heating zones as the main surface, means for feeding through said secondary surface continuously an amount.

of liquid proportional to the amount of operating medium fed through the main heating surface, the flow speed of the liquid through said secondary surface being substantially greater than the flow speed of said operating medium through the main heating surface, and means responsive to the liquid temperature variations in said secondary surface for controlling the heat supply to the generator.

5. In a steam generator with forced passage of the operating medium and having a heat sup-, ply producing several heating zones of different temperatures, a main heating surface for said medium extending through the several heating zones and a secondary heating surface traversed by a suitable fluid and extending at least approximately through the same heating zones as the main surface, means for feeding the fluid through said secondary heating surface at a constant rate independently of the load .fluctuations of the generator, the flow speed of said fluid being substantially greater than the flow speed of said operating medium through the main heating surface, and means responsive to the fluid temperature variations in said secondary surface for controlling the heat supply to the generator.

6. In a steam generator with forced passage of the operating'medium and having a heat supply producing several heating zones of different temperatures, a main heating surface for said medium extending through the several heating zones and a secondary heating surface traversed by a suitable fluid and extending at least approximately through the same heating zones as the main surface, the flow speed of the fiuid through said secondary surface being substantially greater than the flow "speed of said operating medium through the main heating surface, a temperature responsive means disposed at the fluid discharge end ofsaid secondary surface and means actuated by said temperature responsive means for controlling the heat supply of the generator, whereby the steam temperature a the main surface, the flow speed.of the fluid through said secondary surfacerbeing substantially greater than the flow speed of said opervariations.

ating medium through the main heating surface, a temperature responsive means disposed at .the fluid discharge end of'said secondary surface and means actuated by said temperature responsive means for controlling the heat supply of the generator, whereby the steam temperature at the generator outlet is maintained at a given value,- and atemperature "responsive means disposed at the fluid supply end of said of the-operating medium and having a heat supply producing several heating zones of different temperatures, a main heating surface for said medium extending through the several heating zones and a secondary heating surface traversed by a suitable fluid and extending at least approximately through the same heating zones as the main surface, the flow speed of the fluid through said secondary surface being substantially greater than the flow speed of said operating medium through'the main heating surface, a temperature responsive means disposed at the fluid discharge end of said secondary surface and means actuated by said temperature responsive means forcontrolling the heat supply of the generator, whereby the steam temperature at the generator outlet is maintained at a given value, and a temperature responsive means disposed at the medium supply end of said main heating surface and being also connected to said generator heat control means for additionally affecting the heat supply of the generator in accordance with the temperature of the generator operating medium supply to maintain the steam temperature at the generator outlet a said given 'value.

9. In a steam generator with forced passage of the operating medium and having a heat supply producing several heating zones of different temperatures, a main heating surface for said medium extending through the several heating zones and a secondary heating surface traversed by a suitable fluid and extending at least approximately through the same heating zones as the main surface, means for feeding the fluid through said secondary heating surface at a constant rate independently of the load fluctuations of the generator, the flow speed of said fluid being substantially greater than the flow speed of said operating medium through the main heating surface, a temperature responsive means disposed at the fluid discharge end of said secondary surface and means actuated by said temperature responsive means for controlling the heat supply to the generator, whereby the steam temperature at.the generator outlet is maintained at a given value and a governor responsive to the load variations of the generator and being also connected to said generator heat control means so as to compensate for reactions of the temperature responsive means at the outlet of the secondary surface due to variations in the generator load, to'prevent the actuation of said generator heat control means by the temperature variations at the outlet end of the secondary surface incidental to generator load HEINZ RABE. 

